1. Field of the Invention
The various embodiments of the present invention are directed to high temperature circuits for use in high temperature environments. More particularly, at least some embodiments of the invention are directed to high temperature circuits for use in downhole logging tools.
2. Background of the Invention
In the exploration for and extraction of subterranean hydrocarbons, downhole tools are used to determine characteristics of formations traversed by a borehole. In some cases, the downhole tools may be wireline tools which are suspended in a borehole after the drill string has been removed or “tripped.” In other cases, the downhole tools may comprise measuring-while-drilling (MWD) and logging-while-drilling (LWD) tools coupled within the drill string.
Regardless of the mechanism by which a downhole tool is placed within a borehole, the range of temperatures within which the tool must operate may vary widely. For example, a downhole tool placed within a borehole may experience an ambient temperature of approximately 20° C. near the surface, and at depth experience temperatures as high as or exceeding 200° C. Because many of these downhole tools, especially MWD and LWD tools, have onboard electronics which may be semiconductor devices, these semiconductor devices need to be operable over the wide range of temperatures.
Many commercially available semiconductor devices do not operate properly in ambient conditions approaching or exceeding 200° C. Even if a semiconductor device is operational at high temperature, its operating characteristics may change with temperature. For example, logic gates may experience a significant change in the gate delay associated with signal propagation through the gates. Amplifier gain may change as a function of ambient temperature. Charged cell-type memory devices may lose their storage charge more quickly, thus requiring more frequent refresh cycles. While it may be possible to cool downhole devices to be within the normal operating ranges, the implementation of downhole cooling is difficult and extremely inefficient.
Thus, downhole tool manufacturers are relegated to testing the out of specification operating characteristics of commercially available semiconductor devices, using only those devices that still operate in the out of specification temperature conditions, and compensating for changes in operating characteristics caused by excessive temperature swings.